Device and method for detecting direction of polarization of ferroelectric material

ABSTRACT

A device for detecting polarization direction of a ferroelectric is provided. From a measurement signal provided through a probe disposed in contact with or near the surface of a ferroelectric, a demodulation means generates a detection signal having a signal level corresponding to a capacitance change of the ferroelectric due to application of an alternating electric field to a capacitor component formed in the ferroelectric directly below the probe. A synchronous detection means performs synchronous detection of the detection signal based on a synchronous signal and generates a polarization direction detection signal corresponding to the polarization direction of the ferroelectric. A pseudo-noise signal generation means generates a pseudo-noise signal with the same frequency as that of the electric field signal and a different phase and amplitude therefrom. The demodulation means includes a noise component removal means that removes noise components in the measurement signal through signal arithmetic processing with the pseudo-noise signal.

TECHNICAL FIELD

The present invention relates to a device and method for detecting thedirection of polarization of a ferroelectric material.

BACKGROUND ART

Recently, demand for a technology for storing a large amount ofinformation at a high speed has increased along with an increase in theamount of information. The storage density of magnetic recording, whichis currently the most widely used as means for recording information, isapproaching the theoretical limit. Even when vertical magnetic recordingis used, it is believed that 1 Tbit/inch² is the upper storage densitylimit. On the other hand, a ferroelectric exhibits spontaneouspolarization, the direction of which can be reversed by applying anelectric field from the outside to the ferroelectric. Accordingly, it ispossible to record information by associating corresponding digital datawith the direction of polarization of the ferroelectric. In addition, adomain wall of the ferroelectric has a thickness of about 1 or 2 latticeunits and is significantly thinner than that of the ferromagnetic as iswell known. Since the domain size of the ferroelectric is also muchsmaller than that of the ferromagnetic, it is believed that it will bepossible to obtain an ultrahigh-density storage device if it is possibleto control such microscopic domains of the ferroelectric. However, it isdifficult to measure inner polarizations of the ferroelectric, i.e., toread information recorded in the ferroelectric since the innerpolarizations of the ferroelectric are shielded by surface charges onthe ferroelectric such as electrons or ions attached to the surface ofthe ferroelectric.

A Scanning Nonlinear Dielectric Microscope (SNDM) is known as a devicefor purely electrically detecting the distribution of polarization of aferroelectric. FIG. 1 is a block diagram of a conventional device fordetecting the direction of polarization of a ferroelectric, to which theSNDM is applied. This device determines the direction of polarization ofa ferroelectric material 1 by measuring the nonlinear dielectricconstant of the ferroelectric material 1, i.e., capacitance Cp thereofdirectly below a probe 3. In this device, to detect the direction ofpolarization of the ferroelectric material 1, an alternating electricfield Ep is applied between a stage 2 and both a link probe 4 and aprobe 3. Thus, the oscillation frequency of an oscillator 5 changesaccording to the alternating electric field. Since the rate of thechange of the oscillation frequency including the sign is determined bythe nonlinear dielectric constant (i.e., the capacitance Cp) directlybelow the probe, the probe 3 detects the distribution of polarization ofthe ferroelectric material 1 by performing 2D scanning on theferroelectric material 1. After the change of the frequency of theoscillator 5 is demodulated by an FM demodulator 6, the frequency changeis detected through synchronous detection using the frequency of theapplied electric field at a PSK demodulator 7.

Patent Reference 1:

-   Japanese Patent Kokai No. 2004-127489

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In the device constructed as described above, to achieve a high datatransfer rate during data reproduction, there is a need to set theoscillation frequency of the alternating electric field Ep applied tothe ferroelectric material to be high. However, when the high-frequencyalternating electric field is applied to the ferroelectric material,electrodes coupled to the ferroelectric material serve as antennas toeasily emit noise. This noise propagates through the air to reach theoscillator having an inductor component so that the noise component issuperimposed on an output signal of the oscillator, thereby distortingthe signal. Thus, the PSK demodulator does not properly performsynchronous detection, reducing the sensitivity of detection of thedirection of polarization of the ferroelectric material and thus causinga decrease in the accuracy of reproduction of data recorded on theferroelectric material.

Therefore, the present invention has been made in view of the abovecircumstances, and it is an object of the present invention to provide adevice and method for detecting the direction of polarization of aferroelectric material, which can maintain high signal detectionsensitivity by suppressing the influence of noise generated when analternating electric field having a relatively high frequency has beenapplied to the ferroelectric material.

Measure Taken to Solve the Problem

A device for detecting the direction of polarization of a ferroelectricmaterial according to the present invention includes at least one probedisposed in contact with or near a surface of a ferroelectric and anelectric field applying means for providing an electric field signal tothe ferroelectric and applies an alternating electric field to acapacitor component formed in the ferroelectric directly below theprobe, wherein the device detects a direction of polarization of theferroelectric directly below the probe based on a capacitance change ofthe capacitor component as the alternating electric field is applied tothe capacitor component, the device further including a demodulationmeans for generating a detection signal having a signal levelcorresponding to the capacitance change of the ferroelectric as thealternating electric field is applied from a measurement signal providedthrough the probe, a synchronous detection means for performingsynchronous detection of the detection signal based on a synchronoussignal and generates a polarization direction detection signalcorresponding to the polarization direction of the ferroelectric, and apseudo-noise signal generation means for generating a pseudo-noisesignal whose frequency is equal to a frequency of the electric fieldsignal and whose phase and amplitude are different from those of theelectric field signal, wherein the demodulation means includes a noisecomponent removal means for removing a noise component included in themeasurement signal through signal arithmetic processing with thepseudo-noise signal.

In a method for detecting the direction of polarization of aferroelectric material, at least one probe is disposed in contact withor near a surface of a ferroelectric, an electric field signal isprovided to the ferroelectric, an alternating electric field is appliedto a capacitor component formed in the ferroelectric directly below theprobe, and polarization direction of the ferroelectric directly belowthe probe is detected based on a capacitance change of the capacitorcomponent as the alternating electric field is applied to the capacitorcomponent, the method including a demodulation process that generates adetection signal having a signal level corresponding to the capacitancechange of the ferroelectric as the alternating electric field is appliedfrom a measurement signal provided through the probe, a synchronousdetection process that performs synchronous detection of the detectionsignal based on a synchronous signal and generates a polarizationdirection detection signal corresponding to the polarization directionof the ferroelectric, and a pseudo-noise signal generation process thatgenerates a pseudo-noise signal whose frequency is equal to a frequencyof the electric field signal and whose phase and amplitude are differentfrom those of the electric field signal, wherein the demodulationprocess includes a noise component removal process that removes a noisecomponent included in the measurement signal through signal arithmeticprocessing with the pseudo-noise signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a conventionaldetection device;

FIG. 2 is a block diagram illustrating a configuration of a device fordetecting the direction of polarization of a ferroelectric materialaccording to an embodiment of the present invention;

FIG. 3 is a block diagram illustrating a more detailed configuration ofan FM demodulator according to the embodiment of the present invention;

FIG. 4 is a block diagram illustrating a more detailed configuration ofa signal generator according to the embodiment of the present invention;

FIG. 5 is a block diagram illustrating a more detailed configuration ofa synchronous detector according to the embodiment of the presentinvention;

FIG. 6 is a timing chart of each signal generated by the polarizationdirection detection device according to the embodiment of the presentinvention;

FIG. 7 is a block diagram illustrating a configuration of a device fordetecting the direction of polarization of a ferroelectric materialaccording to a second embodiment of the present invention;

FIG. 8 is a block diagram illustrating a configuration of a device fordetecting the direction of polarization of a ferroelectric materialaccording to a third embodiment of the present invention;

FIG. 9 illustrates frequency transfer characteristics of a seriesresonant circuit according to the third embodiment of the presentinvention; and

FIG. 10 is a timing chart of each signal generated by a polarizationdirection detection device according to another embodiment of thepresent invention.

EXPLANATION OF SIGNS

-   10: medium (ferroelectric material)-   11: probe-   20: oscillator-   30: FM demodulator-   40: subtractor-   50: synchronous detector-   60: low pass filter-   70: signal generator-   80: reference phase oscillator-   90: phase comparator-   100: band pass filter

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings. In the drawings described below,substantially the same or equivalent elements or portions are denoted bythe same reference numerals.

First Embodiment

FIG. 2 is a block diagram illustrating a configuration of a device fordetecting the direction of polarization of a ferroelectric materialaccording to the present invention. A medium 10 is a measurement targetof the polarization direction detection device of the present inventionand includes, for example, a ferroelectric material such as LiTaO₃. Thedirection of polarization of the medium 10 can be changed by applying anelectric field greater than a coercive electric field to the medium 10and data can be recorded on the medium 10 by determining the directionof polarization of the medium 10 in association with the data. That is,the polarization direction detection device of the present invention maybe used as a reproduction device that reproduces data recorded on themedium 10 by detecting the polarization direction of the ferroelectricmaterial. The direction of polarization of the medium 10 is reflected inthe nonlinear dielectric constant of the ferroelectric material, i.e.,in the capacitance Cp of a capacitor C formed in the medium 10.

A probe 11 is disposed such that a tip thereof is in contact with ornear the medium 10. The probe 11 detects change of the capacitance Cp ofthe capacitor C directly below the probe 11 due to application of anelectric field signal V3(t) to the medium 10 and reads data recorded onthe probe 11. For example, the movement of the relative position of theprobe 11 and the medium 10 may be achieved by rotating the medium 10 inthe case where the medium 10 is disc-shaped. The movement of therelative position of the probe 11 and the medium 10 may be achieved bylinearly moving either the probe 11 or the medium 10 in the case wherethe medium 10 is card-shaped.

An oscillator 20 includes an inductor L which forms an LC resonantcircuit together with a capacitor C formed directly below the probe 11and generates an oscillation signal V1(t) frequency-modulated throughchange of the capacitance Cp of the capacitor C. The oscillator 20 isdesigned such that the capacitance Cp is on the order of picofarads(pF), the inductance of the inductor L is on the order of nanohenries,and the frequency of the oscillation signal V1(t) is in a range fromhundreds of MHz to several GHz. Any oscillator, in which an oscillationloop, which includes the probe 11 and the capacitor C and generates theoscillation signal V1(t) according to the capacitance Cp of thecapacitor C, is formed may be used as the oscillator 20.

The FM demodulator 30 converts the oscillation signal V1(t) into a lowfrequency signal by mixing the oscillation signal V1(t) with a localoscillation signal and generates a frequency detection signal V2(t)having a voltage level according to the frequency f1 of the oscillationsignal V1(t). The frequency detection signal V2(t) is provided to thesubtractor 40. A more detailed configuration of the FM demodulator 30will be described later.

The signal generator 70 generates an electric field signal V3(t) havinga frequency. The electric field signal V3(t) is provided to the rearside of the medium 10. It can be assumed that the probe 11 is groundedat the frequency fe of the electric field signal V3(t) since theinductance of the inductor L of the oscillator 20 is sufficiently lowand one end of the inductor L is grounded at the frequency fe.Accordingly, when the electric field signal V3(t) is applied to the rearside of the medium 10, an alternating electric field is applied betweenthe medium 10 and the probe. The nonlinear dielectric constant of themedium 10 changes as an alternating electric field is applied to themedium 10. The capacitance Cp of the capacitor C directly below theprobe 11 changes accordingly. The manner (or form) of change of thecapacitance Cp as the alternating electric field is applied variesdepending on the polarization state of the medium 10. Specifically, whenthe capacitance of the capacitor C is represented by “Cpp” when thepolarity of the electric field signal V3(t) is positive and “Cpn” whenthe polarity of the electric field signal V3(t) is negative, themagnitude relationship between Cpp and Cpn is reversed according to thepolarization direction of the medium 10. Namely, whether the capacitanceCp directly below the probe 11 increases or decreases as the polarity ofthe electric field signal V3(t) changes depends on the polarizationdirection of the medium 10. The polarization direction detection deviceof the ferroelectric material according to the present invention detectschange of the capacitance Cp which is based on the applied electricfield signal V3(t) to achieve detection of the polarization direction ofthe medium 10, i.e., reproduction of data recorded on the medium 10. Theamount of change of the capacitance Cp as the electric field signalV3(t) is applied is on the order of attofarads (aF: 10⁻¹⁸ F) and thus itis possible to detect a very small capacitance change. The frequency feof the electric field signal V3(t) is sufficiently smaller than thefrequency f1 of the oscillation signal V1(t) and is set, for example, ina range from several KHz to hundreds of KHz and satisfies a relation off1>>fe. The signal generator 70 generates a synchronous signal V4(t)which has the same frequency as the electric field signal V3(t) and hasa predetermined delay time relative to the electric field signal V3(t)and provides the synchronous signal V4(t) to the synchronous detector50. The signal generator 70 generates a cancel signal V5(t) which hasthe same frequency as the electric field signal V3(t) and has adifferent amplitude and phase from the electric field signal V3(t) andprovides the cancel signal V5(t) to the subtractor 40. The cancel signalV5(t) is a signal whose phase and amplitude have been adjusted toapproximately reproduce noise emitted from the medium 10 as analternating electric field is applied to the medium 10. A detailedconfiguration of the signal generator 70 will be described later.

The subtractor 40 performs a signal calculation process for subtractingthe cancel signal V5(t) from the frequency detection signal V2(t)provided from the FM demodulator 30. This removes the noise componentincluded in the frequency detection signal V2(t) due to application ofan alternating electric field to the medium 10. The subtractor 40obtains a corrected signal V6(t) through such a signal calculationprocess and provides the corrected signal V6(t) to the synchronousdetector 50.

The synchronous detector 50 performs synchronous detection of thecorrected signal V6(t) using the synchronous signal V4(t) and outputsthe resulting signal as a synchronous-detected signal V7(t) and providesthe synchronous-detected signal V7(t) to the low pass filter 60. Adetailed configuration of the synchronous detector 50 will be describedlater.

The low pass filter 60 removes the component of the frequency fe of theapplied electric field, harmonic components, etc., from thesynchronous-detected signal V7(t) to generate a reproduced signal V8(t).The reproduced signal V8(t) has a signal level according to thepolarization direction of the medium 10. Accordingly, the polarizationdirection of the medium 10 is detected by generating the reproducedsignal V8(t).

FIG. 3 is a block diagram illustrating a more detailed configuration ofthe FM demodulator 30. The FM demodulator 30 includes a mixer 31, an f-vconverter 32, a subtractor 33, a band pass filter 34, a controller 35,and a voltage controlled oscillator 36.

The mixer 31 includes, for example, a double-balanced mixer, a low passfilter, and an amplifier, all of which are not shown. Thedouble-balanced mixer mixes the oscillation signal V1(t) having thefrequency f1 provided from the oscillator 20 and the local oscillationsignal having the frequency f2 provided from the voltage controlledoscillator 36 to generate two beat signals having different frequencies.That is, the double-balanced mixer generates a first beat signal havinga frequency represented by “f1+f2” and a second beat signal having afrequency represented by |f1−f2|. The low pass filter (not shown)removes the first beat signal whose frequency is high and passes thesecond beat signal whose frequency is low. The second beat signal is asignal represented by |f1−f2| as described above and has a frequencycorresponding to the difference between the frequency f1 of theoscillation signal V1(t) and the frequency f2 of the local oscillationsignal. Accordingly, the frequency of the second beat signal is lowerthan the frequency of the oscillation signal V1(t). The second beatsignal is amplified by the amplifier (not shown) and is output as afrequency-converted signal. That is, the mixer 31 outputs, as afrequency-converted signal Δf, a low frequency signal obtained throughfrequency-conversion of the oscillation signal V1(t) provided from theoscillator 20.

The f-v converter 32 generates an f-v converted signal having a voltagelevel proportional to the frequency |f1−f2| of the frequency-convertedsignal Δf generated by the mixer 31. The f-v converter 32 includes, forexample, a comparator, a monostable multivibrator, a low pass filter,and an amplifier, all of which are not shown. The comparator comparesthe frequency-converted signal Δf with a predetermined reference leveland outputs a digital value of “1” when the signal level of thefrequency-converted signal Δf is higher than the reference level andoutputs a digital value of “0” when the signal level of thefrequency-converted signal Δf is lower than the reference level. Themonostable multivibrator generates a sequence of pulse signals having auniform pulse width through triggering at rising edges of the binarysignal from the comparator. The pass band of the low pass filter is setso as to remove the frequency of the frequency-converted signal Δf as acarrier component. The low pass filter averages the pulse sequenceoutput from the monostable multivibrator. Through such signalprocessing, the f-v converter 32 converts the frequency-converted signalΔf provided from the mixer 31 into a voltage signal according to thefrequency of the frequency-converted signal Δf. The f-v converted signalfrom the f-v converter 32 is provided to the subtractor 33 and the bandpass filter 34.

The band pass filter 34 has a pass band, the central frequency of whichis set to the frequency fe of the electric field signal V3(t), andremoves, as unnecessary signal components, components (for example, anoise component such as a hum) included in the f-v converted signalother than components changed due to application of an alternatingelectric field and outputs the resulting signal as a frequency detectionsignal V2(t) which is the output signal of the FM demodulator 30.

The subtractor 33 receives a target frequency signal and the f-vconverted signal from the f-v converter 32. The target frequency signalrepresents a target value of the f-v converted signal. The subtractor 33subtracts the target frequency signal from the f-v converted signal andoutputs the resulting signal as an error signal. That is, the errorsignal corresponds to the difference between the f-v converted signaland the target value. The error signal generated by the subtractor 33 isprovided to the controller 35.

For example, the controller 35 includes an inverting integrator thatintegrates the error signal provided from the subtractor 33 and performsphase compensation and phase reversal on the error signal so that thevalue of the f-v converted signal is equal to the target value andoutputs the resulting signal as a control signal. That is, thecontroller 35 increases the output level of the control signal when theerror signal is negative and decreases the output level of the controlsignal when the error signal is positive. The control signal is providedto the voltage controlled oscillator 36.

The voltage controlled oscillator 36 includes, for example, an inductor,a variable capacitance diode, and an active element, all of which arenot shown, and changes the capacitance of the variable capacitance diodeaccording to the control signal provided from the controller 35. As aresult, the voltage controlled oscillator 36 outputs a local oscillationsignal that oscillates at the frequency f2 according to the voltagelevel of the control signal.

As described above, the FM demodulator 30 forms a feedback control loopthrough the mixer 31, the f-v converter 32, the subtractor 33, thecontroller 35, and the voltage controlled oscillator 36 to control thefrequency of the local oscillation signal so that the f-v convertedsignal matches the target frequency signal. For example, even when thecapacitance Cp of the capacitor C directly below the probe 11 hasgreatly changed due to movement of the probe 11 above the medium 10 andthe frequency of the oscillation signal V1(t) has greatly changedaccordingly, such feedback control allows the local oscillation signalto change following the change of the frequency of the oscillationsignal V1(t), so that the output signal of the f-v converter 32 and thefrequency-converted signal Δf are uniform. Thus, the change of thefrequency due to the change of the data reproduction position of themedium 10 is removed, thereby enabling highly accurate signal detection.More specifically, when the oscillation frequency of the oscillationsignal V1(t) is, for example, 1 GHz, the frequency change of theoscillation signal V1(t) due to change of the data reproduction positionmay exceed 1 MHz. If the frequency f2 of the local oscillation signal isset to a fixed value without performing feedback control in the casewhere a frequency of several hundreds of KHz is selected as thefrequency of the frequency-converted signal Δf (=|f1−f2|) output fromthe mixer 31, the amount of change of the frequency f1 of theoscillation signal V1(t) due to change of the reproduction position ofthe medium 10 exceeds the frequency of the frequency-converted signalΔf, resulting in failure of frequency detection. Therefore, in thepresent invention, a feedback control loop is formed to control thefrequency of the local oscillation signal so as to follow the frequencydeviation of the oscillation signal V1(t) so that thefrequency-converted signal Δf and the f-v converted signal fall within aspecific range, so that it is possible to perform reliable (or stable)frequency detection even when the capacitance Cp directly below theprobe 11 has greatly changed due to change of the reproduction positionor the like.

FIG. 4 illustrates a more detailed configuration of the signal generator70. In the signal generator 70, a quartz oscillator 71 generates a clocksignal at a stable oscillation frequency of, for example, 10 MHz andprovides the clock signal to a frequency divider 72 and phase adjusters75 and 76 which are described later. The frequency divider 72 dividesthe frequency of the input clock signal by, for example, 1000 togenerate a reference frequency signal having a frequency of 10 KHz andprovides the reference frequency signal to a band pass filter 73 and thephase adjusters 75 and 76. The band pass filter 73 has a pass band whosecentral frequency is the frequency fe (for example, 10 KHz) of theelectric field signal V3(t) and shapes the reference frequency signalhaving a rectangular waveform provided from the frequency divider 72into a sinusoidal wave. Since the reference frequency signal having arectangular waveform includes various frequency components at edgeportions thereof, applying the reference frequency signal without changeas the electric field signal V3(t) to the medium 10 is not desirable toachieve highly-accurate signal detection. Therefore, in this embodiment,the reference frequency signal is passed through the band pass filter 73to convert the same into a sinusoidal signal having a single frequencycomponent to increase signal detection sensitivity. The amplitudeadjuster 74 adjusts the amplitude and offset voltage of the referencefrequency signal having a sinusoidal waveform to generate an electricfield signal V3(t) having an amplitude of ±5V and a frequency of, forexample, 10 KHz and provides the electric field signal V3(t) to themedium 10. Through the operation of the amplitude adjuster 74, the levelof the electric field signal V3(t) is adjusted and an appropriateintensity of alternating electric field is applied to the medium 10.Specifically, the amplitude adjuster 74 adjusts the amplitude level ofthe electric field signal V3(t) to an amplitude level which is requiredto read data recorded on the medium 10 and is also less than thatrequired to write data to the medium 10.

The phase adjuster 75 includes a shift register and generates asynchronous signal V4(t) by shifting the phase of the referencefrequency signal according to the clock signal. That is, the phaseadjuster 75 delays the electric field signal V3(t) by a time Tdcorresponding to the amount of delay from the output of the electricfield signal V3(t) to the synchronous detection by the synchronousdetector 50 and outputs the delayed electric field signal V3(t) as thesynchronous signal V4(t). The synchronous signal V4(t) is provided tothe synchronous detector 50.

Similarly, the phase adjuster 76 includes a shift register and shiftsthe phase of the reference frequency signal according to the clocksignal to add a predetermined delay time to the reference frequencysignal. A band pass filter 77 has a pass band whose central frequency isthe frequency fe (for example, 10 KHz) of the electric field signalV3(t) and shapes the reference frequency signal having a rectangularwaveform, the phase of which has been adjusted by the phase adjuster 76,into a sinusoidal waveform having the same single frequency component asthe electric field signal V3(t). An amplitude adjuster 78 adjusts theamplitude and offset voltage of the output signal of the band passfilter 77 and outputs the resulting signal as a cancel signal V5(t). Thecancel signal V5(t) has the same form and frequency as the electricfield signal V3(t) and has a different phase and amplitude from theelectric field signal V3(t). Through phase and amplitude adjustment bythe phase adjuster 76 and the amplitude adjuster 78, the cancel signalV5(t) can approximately reproduce a noise component that is emitted fromthe medium 10 due to application of a high-frequency alternatingelectric field to the medium 10 and is superimposed on the oscillationsignal V1(t) or the like. Since the noise component is generated due toapplication of an alternating electric field to the medium 10, the noisecomponent has the same frequency component as the electric field signalV3(t) and the phase and amplitude of the noise component are stabilizedif the configuration, arrangement, etc., of the oscillator 20, themedium 10, and the like are fixed. Accordingly, it is possible toapproximately reproduce the noise component by adjusting the phase andamplitude of the electric field signal V3(t) while keeping the frequencyof the electric field signal V3(t) unchanged as described above. In thepresent invention, the signal generator 70 generates the approximatelyreproduced pseudo-noise signal as the cancel signal V5(t). Since thephase and amplitude of the noise component are stable and rarely vary ifthe configuration, arrangement, etc., of the oscillator 20, the medium10, and the like are fixed as described above, a fixed cancel signalV5(t) may be employed. However, a mechanism for adjusting the phase andamplitude of the noise component may also be provided in considerationof factors causing changes in the noise component, for example,temperature change.

The subtractor 40 subtracts the cancel signal V5(t) from the frequencydetection signal V2(t) output from the FM demodulator 30 to remove anoise component included in the frequency detection signal V2(t) andoutputs the resulting signal as a corrected signal V6(t). Although thesubtractor 40 for subtracting the cancel signal V5(t) is provided toremove the noise component from the frequency detection signal V2(t) inthis embodiment, an adder may also be provided. In this case, there is aneed to generate a cancel signal by reversing the polarity of the cancelsignal described above.

FIG. 5 illustrates a more detailed configuration of the synchronousdetector 50. The synchronous detector 50 includes a polarity reversalunit 51 and an analog switch 52. The corrected signal V6(t) providedfrom the subtractor 40 is input to each of the polarity reversal unit 51and the analog switch 52. The polarity reversal unit 51 reverses thepolarity of the corrected signal V6(t) and provides the resulting signalto the analog switch 52. That is, both a signal obtained by reversingthe polarity of the corrected signal V6(t) and the corrected signalV6(t) which has the original polarity since it has not passed throughthe polarity reversal unit 51 are input to the analog switch 52. Thesynchronous signal V4(t) generated by the signal generator 70 is alsoinput to the analog switch 52. The analog switch 52 uses the synchronoussignal V4(t) as a control signal and outputs the corrected signal V6(t),which has not been subjected to the reversal process, as asynchronous-detected signal V7(t), for example, when the synchronoussignal V4(t) is at a high level and outputs the corrected signal V6(t),which has been subjected to the reversal process, as asynchronous-detected signal V7(t), for example, when the synchronoussignal V4(t) is at a low level. That is, the analog switch 52 forms aso-called chopper circuit and detects only a component of the correctedsignal V6(t) which is synchronized with the synchronous signal V4(t) andoutputs the detected component as the synchronous-detected signal V7(t).

Next, the operation of the device for detecting the direction ofpolarization of a ferroelectric material according to the presentinvention is described with reference to a timing chart shown in FIG. 6.In FIG. 6, sections 1 and 2 represent polarization domains of the medium10 and data “1” is recorded in section 1 and data “0” is recorded insection 2. That is, in sections 1 and 2, the medium 10 exhibitsdifferent polarization states corresponding to the respective data ofthe sections 1 and 2.

The signal generator 70 applies an electric field signal V3(t) having asinusoidal waveform, the polarity of which periodically changes as shownin FIG. 6, to the medium 10. Accordingly, an alternating electric fieldis applied to a capacitor C directly below the probe 11 of the medium 10and the capacitance Cp of the capacitor C changes according to thepolarity of the applied alternating electric field. Here, an electricfield having a positive direction is applied to the medium 10 when thepolarity of the electric field signal V3(t) is positive. Let Cpp be thecapacitance of the capacitor C at this time. On the other hand, anelectric field having a negative direction is applied to the medium 10when the polarity of the electric field signal V3(t) is negative. LetCpn be the capacitance of the capacitor C at this time. As describedabove, the polarization directions of the medium 10 in sections 1 and 2are different. Accordingly, a relation of Cpp<Cpn is satisfied insection 1 and a relation of Cpp>Cpn is satisfied in section 2.Therefore, in section 1, the oscillation frequency of the oscillationsignal V1(t) output from the oscillator 20 when an electric field havinga positive direction is applied is higher than when an electric fieldhaving a negative direction is applied. On the other hand, in section 2,the oscillation frequency of the oscillation signal V1(t) output fromthe oscillator 20 when an electric field having a positive direction isapplied is lower than when an electric field having a negative directionis applied. Thus, the oscillator 20 converts a change of the capacitanceCp due to application of an alternating electric field into a frequencychange and outputs the resulting signal as the oscillation signal V1(t).

The FM demodulator 30 converts a change of the frequency of theoscillation signal V1(t) due to application of an alternating electricfield into a voltage change and outputs the resulting signal as afrequency detection signal V2(t). However, when the frequency of theelectric field signal V3(t) is relatively high, the electrodes of themedium 10 serve as antennas emitting noise and the emitted noise isreceived by the FM demodulator 30. As a result, the noise component issuperimposed on the oscillation signal V1(t) and the frequency detectionsignal V2(t) output from the FM demodulator 30 is distorted as shown inFIG. 6. The distorted frequency detection signal V2(t) and the cancelsignal V5(t) generated by the signal generator 70 are input to thesubtractor 40. The subtractor 40 subtracts the cancel signal V5(t) fromthe distorted frequency detection signal V2(t) to suppress the distortedcomponent and outputs the resulting signal as a corrected signal V6(t).The phase and amplitude of the cancel signal V5(t) are adjusted so as tominimize the distorted component of the corrected frequency detectionsignal V2(t). The frequency detection signal V2(t) from which thedistorted component has been removed, i.e., the corrected signal V6(t)is delayed relative to the electric field signal V3(t) by a time Td. Insection 1, the corrected signal V6(t) exhibits a high level in responseto application of an electric field having a positive direction andexhibits a low level in response to application of an electric fieldhaving a negative direction. On the other hand, in section 2, thecorrected signal V6(t) exhibits signal levels opposite to those insection 1, i.e., exhibits a low level in response to application of anelectric field having a positive direction and exhibits a high level inresponse to application of an electric field having a negativedirection. The signal generator 70 generates a synchronous signal V4(t)that is delayed from the time when the electric field signal V3(t) isoutput by the time Td corresponding to the amount of delay fromapplication of the electric field signal V3(t) to the synchronousdetection and provides the synchronous signal V4(t) to the synchronousdetector 50. As a result, the corrected signal V6(t) is in phase withthe synchronous signal V4(t) in section 1 and is antiphase to thesynchronous signal V4(t) in section 2.

The analog switch 52 included in the synchronous detector 50 uses thesynchronous signal V4(t) as a control signal, and directly outputs thecorrected signal V6(t), which has not been subjected to the reversalprocess since it has not passed through the polarity reversal unit 51,to generate the synchronous-detected signal V7(t) when the synchronoussignal V4(t) is at a high level and outputs the corrected signal V6(t)after reversing the polarity thereof through the polarity reversal unit51 to generate the synchronous-detected signal V7(t) when thesynchronous signal V4(t) is at a low level. That is, in section 1, thesynchronous detector 50 outputs, as a synchronous-detected signal V7(t),the corrected signal V6(t) without performing the reversal process onthe corrected signal V6(t) when the corrected signal V6(t) is at a highlevel and outputs, as a synchronous-detected signal V7(t), the correctedsignal V6(t) after performing the reversal process on the correctedsignal V6(t) when the corrected signal V6(t) is at a low level. On theother hand, in section 2, the synchronous detector 50 outputs, as asynchronous-detected signal V7(t), the corrected signal V6(t) afterperforming the reversal process on the corrected signal V6(t) when thecorrected signal V6(t) is at a high level and outputs, as asynchronous-detected signal V7(t), the corrected signal V6(t) withoutperforming the reversal process on the corrected signal V6(t) when thecorrected signal V6(t) is at a low level. The synchronous-detectedsignal V7(t) obtained through such signal processing of the synchronousdetector 50 is only positive in polarity in section 1 and is onlynegative in polarity in section 2.

The low pass filter 60 removes a carrier component from thesynchronous-detected signal V7(t) to generate a reproduced signal V8(t).The reproduced signal V8(t) exhibits a high level in section 1 andexhibits a low level in section 2. That is, data “1” and “0” recorded onthe medium 10 are output as reproduced signals V8(t) at differentvoltage levels and are thus reproduced as purely electrical signals.Namely, the polarization direction of the ferroelectric material isdetected purely electrically.

As described above, the device for detecting the direction ofpolarization of a ferroelectric material according to the presentinvention generates a cancel signal V5(t) which has the same shape andfrequency as the electric field signal V3(t) for applying an alternatingelectric field to the medium 10 and a different phase and amplitude fromthe electric field signal V3(t). Through adjustment of the phase andamplitude of the cancel signal V5(t), the cancel signal V5(t) canapproximately reproduce a noise component, which is emitted from themedium 10 when a high-frequency alternating electric field is applied tothe medium 10 and is then superimposed on the oscillation signal V1(t).In the device for detecting the direction of polarization of aferroelectric material according to the present invention, apseudo-noise signal which approximately reproduces such a noisecomponent is generated as the cancel signal V5(t) to remove the noisecomponent through feedforward control. Accordingly, it is possible tomaintain a high signal detection sensitivity even when an alternatingelectric field having a relatively high frequency is applied to detectthe polarization direction of a medium formed of a ferroelectricmaterial.

Second Embodiment

FIG. 7 illustrates a second embodiment of the device for detecting thedirection of polarization of a ferroelectric material of the presentinvention. The detection device of the second embodiment is differentfrom the first embodiment with regard to a signal that is subjected tocorrection by the cancel signal V5(t). That is, in the detection deviceof the first embodiment, the subtractor 40 is provided downstream of theFM demodulator 30 and the frequency detection signal V2(t) output fromthe FM demodulator 30 is subjected to correction by the cancel signalV5(t). In this embodiment, a signal correction process is performed in afrequency control loop of an FM demodulator 30. That is, in thedetection device of this embodiment, a subtractor 40 is provideddownstream of a controller 35 which is provided in the frequency controlloop of the FM demodulator 30 and a control signal from the controller35 and a cancel signal V5(t) from a signal generator 70 are input to asubtractor 40 a. The subtractor 40 a subtracts the cancel signal V5(t)from the control signal provided from the controller 35 and provides theresulting signal as a corrected signal V6(t) to a voltage controlledoscillator 36. The voltage controlled oscillator 36 generates afrequency-modulated local oscillation signal using both the cancelsignal V5(t) and a control error which is based on a normal feedbackcontrol loop and provides the frequency-modulated local oscillationsignal to a mixer 31. In this embodiment, a signal which has the samefrequency and waveform as the electric field signal V3(t) and has adifferent phase and amplitude from the electric field signal V3(t) canalso be used as the cancel signal V5(t). The mixer 31 mixes theoscillation signal V1(t) frequency-modulated using noise emitted fromthe medium 10 due to application of a high-frequency alternatingelectric field and the local oscillation signal frequency-modulatedusing the cancel signal V5(t) and generates a frequency-converted signalΔf. Here, by appropriately adjusting the phase and amplitude of thecancel signal V5(t), it is possible to obtain a frequency-convertedsignal Δf from which the noise component has been removed. Thesubsequent processes are similar to those of the first embodiment.Accordingly, the noise component can also be removed by performingsignal correction using the cancel signal V5(t) in the frequency controlloop of the FM demodulator 30, thereby achieving the same advantages asthe first embodiment.

Third Embodiment

FIG. 8 illustrates a third embodiment of the device for detecting thedirection of polarization of a ferroelectric material of the presentinvention. The detection device of the third embodiment is differentfrom the first and second embodiments with regard to the principle ofdetection of the polarization direction of the medium 10. That is, thedetection device of the first and second embodiments converts a changeof the capacitance Cp of the capacitor C formed directly below the probe11 due to application of an alternating electric field into a frequencychange using an oscillator which has the capacitor C as a component anddemodulates the frequency change using an FM demodulator to detect thepolarization direction of the medium 10. The detection device accordingto this embodiment converts a change of the capacitance Cp into a phasechange to detect the polarization direction of the medium 10.Specifically, when compared to the detection device of the firstembodiment, the oscillator 20 and the FM demodulator 30 are removed fromthe detection device of this embodiment and a reference phase oscillator80, a phase comparator 90, a band pass filter 100, and an inductor L areadded as new components to the detection device. The detection device ofthis embodiment is described below, focusing on portions different fromthose of the detection device of the first embodiment.

A probe 11 is disposed such that a tip thereof is in contact with ornear the medium 10. The probe 11 detects a change of the capacitance Cpof the capacitor C directly below the probe 11 due to application of anelectric field signal V3(t) to the medium 10 and reads data recorded onthe probe 11. The inductor L is connected in series to the probe 11.Accordingly, a series resonant circuit including the inductor L and thecapacitor C formed directly below the probe 11 is formed.

The reference phase oscillator 80 generates a reference phase signalV10(t) which oscillates at a resonant frequency f0 of the seriesresonant circuit and provides the reference phase signal V10(t) to theseries resonant circuit and a phase comparator 90. The reference phasesignal V10(t) applied to the series resonant circuit is then output as aresonant signal V11(t) from a connection point between the inductor Land the probe 11 (or the capacitor C) and the resonant signal V11(t) isprovided to the phase comparator 90. The output impedance of thereference phase oscillator 80 and the signal generator 70 issufficiently lower than the impedance of the inductor L and thecapacitor C at the resonant frequency f0. As a result, a series resonantcircuit having a high Q value is formed through the inductor L and thecapacitor C.

The phase comparator 90 generates an output signal having a signal levelcorresponding to the phase difference between the reference phase signalV10(t) and the resonant signal V11(t) and provides the output signal tothe band pass filter 100. The phase comparator 90 may be constructed of,for example, a double-balanced mixer and outputs a DC voltagecorresponding to the phase difference between two signals, which areinput to the phase comparator 90 for operation as a multiplier, whenoscillation frequencies of the two input signals are equal.

The band pass filter 100 has a pass band whose central frequency is thefrequency fe of the electric field signal V3(t) output from the signalgenerator 70 and extracts only a frequency component for application ofan electric field from the output signal of the phase comparator 90 andoutputs the frequency component as a phase difference signal V12(t). Thephase difference signal V12(t) is provided to the subtractor 40.

Similar to the first embodiment, the signal generator 70 generates andprovides an electric field signal V3(t), a cancel signal V5(t), and asynchronous signal V4(t) to the medium 10, the subtractor 40, and thesynchronous detector 50, respectively. The frequency fe of the electricfield signal V3(t) is set to be sufficiently lower than the frequency ofthe reference phase signal V10(t).

The subtractor 40 subtracts the cancel signal V5(t) from the phasedifference signal V12(t) provided from the band pass filter 100 andoutputs the resulting signal as a corrected signal V6(t). Accordingly, anoise component that is emitted from the medium 10 due to application ofa high-frequency alternating electric field to the medium 10 and is thensuperimposed on the reference phase signal V10(t) or the like is removedfrom the phase difference signal V12(t), which has been distorted due tothe noise component, to generate a non-distorted corrected signal V6(t).The corrected signal V6(t) is provided to the synchronous detector 50.The synchronous detector 50 performs synchronous detection of thecorrected signal V6(t) using the synchronous signal V4(t) and outputsthe resulting signal as a synchronous-detected signal V7(t) and providesthe synchronous-detected signal V7(t) to the low pass filter 60. The lowpass filter 60 removes the component of the frequency fe of the appliedelectric field, harmonic components, etc., from the synchronous-detectedsignal V7(t) to generate a reproduced signal V8(t).

FIG. 9 illustrates frequency transfer characteristics of a seriesresonant circuit formed through an inductor L and a capacitor C. Asshown in FIG. 9, gain is peaked and phase rapidly rotates at a resonantfrequency f0 of the series resonant circuit. Here, if an alternatingelectric field is applied to the medium 10, the capacitance Cp of thecapacitor C formed directly below the probe changes. The manner (orform) of change of the capacitance Cp varies depending on thepolarization direction of the medium 10, i.e., data recorded on themedium 10 as described above. For example, in the case where thecapacitance Cp when the polarity of the applied electric field ispositive is represented by “Cpp” (i.e., Cp=Cpp) and the capacitance Cpwhen the polarity of the applied electric field is negative isrepresented by “Cpn” (i.e., Cp=Cpn), Cpp<Cpn when data recorded on themedium 10 is “1” and Cpp>Cpn when data recorded on the medium 10 is “0”.FIG. 9 illustrates transfer characteristics when the data recorded onthe medium 10 is “1” and a relation of Cpp<Cpn is satisfied. In FIG. 9,transfer characteristics when the applied electric field is positive(Cp=Cpp) are shown by a solid line and transfer characteristics when theapplied electric field is negative (Cp=Cpn) are shown by a dotted line.In this case, since Cpp<Cpn, the resonant frequency of the seriesresonant circuit when the electric field is negative is lowered by Ofand the phase of the reference phase signal V10(t) at the resonantfrequency f0 is delayed by ΔΦ, compared to when the applied electricfield is positive. That is, the phase of the resonant signal V11(t) atthe resonant frequency f0 is changed by ΔΦ when the polarity of thealternating electric field switches between positive and negative. Onthe other hand, even when the data recorded on the medium 10 is “0”, aphase change occurs as an alternating electric field is applied. In thiscase, the phase at the resonant frequency f0 when the applied electricfield is negative leads by ΔΦ, compared to when the applied electricfield is positive. That is, by detecting a change in the phase of theresonant signal V11(t) when an alternating electric field is applied, itis possible to detect the polarization direction of the medium 10 andthus to reproduce the recorded data. The polarization directiondetection device according to the present invention detects a change inthe phase of the resonant signal V11(t) through comparison with thephase of the reference phase signal V10(t) to perform detection of thepolarization direction of the medium 10, i.e., reproduction of therecorded data.

Next, the operation of the polarization direction detection deviceaccording to this embodiment is described with reference to a timingchart shown in FIG. 10. In FIG. 10, sections 1 and 2 representpolarization domains of the medium 10 and data “1” is recorded insection 1 and data “0” is recorded in section 2. That is, in sections 1and 2, the medium 10 exhibits different polarization statescorresponding to the respective data of the sections 1 and 2. The signalgenerator 70 applies an electric field signal V3(t) having a sinusoidalwaveform, the polarity of which periodically changes as shown in FIG.10, to the medium 10. Accordingly, an alternating electric field isapplied to a capacitor C directly below the probe 11 of the medium 10and the capacitance Cp of the capacitor C changes according to thepolarity of the applied alternating electric field. Here, an electricfield having a positive direction is applied to the medium 10 when theelectric field signal V3(t) is positive in polarity. Let Cpp be thecapacitance of the capacitor C at this time. On the other hand, anelectric field having a negative direction is applied to the medium 10when the electric field signal V3(t) is negative in polarity. Let Cpn bethe capacitance of the capacitor C at this time. As described above, thepolarization directions of the medium 10 in sections 1 and 2 aredifferent. Accordingly, a relation of Cpp<Cpn is satisfied in section 1and a relation of Cpp>Cpn is satisfied in section 2. Therefore, as thepolarity of the applied alternating electric field is reversed, theresonant frequency of the series resonant circuit changes and thus thephase of the resonant signal V11(t) changes according to the polarity ofthe applied alternating electric field. Specifically, in section 1, thephase of the resonant signal V11(t) when an electric field having anegative direction is applied to the medium 10 is delayed compared towhen an electric field having a positive direction is applied. On theother hand, in section 2, the phase of the resonant signal V11(t) whenan electric field having a positive direction is applied to the medium10 is delayed compared to when an electric field having a negativedirection is applied. The phase comparator 90 generates an output signalhaving a signal level corresponding to the phase difference between thereference phase signal V10(t) and the resonant signal V11(t). However,since the phase of the reference phase signal V10(t) does not change,the output signal of the phase comparator 90 has a level correspondingto a change in the phase of the resonant signal V11(t). The band passfilter 100 has a pass band, the central frequency of which is set to thefrequency fe of the electric field signal V3(t). The band pass filter100 extracts only components which have changed due to application of analternating electric field from the output signal of the phasecomparator 90 and removes other frequency components as noise componentsand outputs the resulting signal as a phase difference signal V12(t).Through such signal processing of the phase comparator 90 and the bandpass filter 100, in section 1, the phase difference signal V12(t)exhibits a high level in response to application of an electric fieldhaving a positive direction to the medium 10 and exhibits a low level inresponse to application of an electric field having a negativedirection. On the other hand, in section 2, the phase difference signalV12(t) exhibits a low level in response to application of an electricfield having a positive direction to the medium 10 and exhibits a highlevel in response to application of an electric field having a negativedirection.

Here, when the frequency of the electric field signal V3(t) applied tothe medium 10 is relatively high, a high-frequency alternating electricfield is applied to the medium 10 so that noise is emitted from themedium 10. The noise is superimposed on the resonant signal V11(t) andthe phase difference signal V12(t) is distorted as shown in FIG. 10. Thedistorted phase difference signal V12(t) and the cancel signal V5(t)generated by the signal generator 70 are input to the subtractor 40. Thesubtractor 40 subtracts the cancel signal V5(t) from the distorted phasedifference signal V12(t) to suppress the distorted component and outputsthe resulting signal as a corrected signal V6(t). The phase andamplitude of the cancel signal V5(t) are adjusted so as to minimize thedistorted component of the corrected phase difference signal V12(t).

The signal generator 70 generates a synchronous signal V4(t) that isdelayed from the time when the electric field signal V3(t) is output bythe time Td corresponding to the amount of delay from application of theelectric field signal V3(t) to the synchronous detection and providesthe synchronous signal V4(t) to the synchronous detector 50. As aresult, the corrected signal V6(t) is phase with the synchronous signalV4(t) in section 1 and is antiphase to the synchronous signal V4(t) insection 2.

The analog switch 52 included in the synchronous detector 50 uses thesynchronous signal V4(t) as a control signal, and directly outputs thecorrected signal V6(t), which has not been subjected to the reversalprocess since it has not passed through the polarity reversal unit 51,to generate the synchronous-detected signal V7(t) when the synchronoussignal V4(t) is at a high level and outputs the corrected signal V6(t)after reversing the polarity thereof through the polarity reversal unit51 to generate the synchronous-detected signal V7(t) when thesynchronous signal V4(t) is at a low level. That is, in section 1, thesynchronous detector 50 outputs, as a synchronous-detected signal V7(t),the corrected signal V6(t) without performing the reversal process onthe corrected signal V6(t) when the corrected signal V6(t) is at a highlevel and outputs, as a synchronous-detected signal V7(t), the correctedsignal V6(t) after performing the reversal process on the correctedsignal V6(t) when the corrected signal V6(t) is at a low level. On theother hand, in section 2, the synchronous detector 50 outputs, as asynchronous-detected signal V7(t), the corrected signal V6(t) afterperforming the reversal process on the corrected signal V6(t) when thecorrected signal V6(t) is at a high level and outputs, as asynchronous-detected signal V7(t), the corrected signal V6(t) withoutperforming the reversal process on the corrected signal V6(t) when thecorrected signal V6(t) is at a low level. The synchronous-detectedsignal V7(t) obtained through such signal processing of the synchronousdetector 50 is only positive in polarity in section 1 and is onlynegative in polarity in section 1 as shown in FIG. 10.

The low pass filter 60 removes a carrier component from thesynchronous-detected signal V7(t) to generate a reproduced signal V8(t).The reproduced signal V8(t) exhibits a high level in section 1 andexhibits a low level in section 2. That is, data “1” and “0” recorded onthe medium 10 are detected as different voltage levels and are thusreproduced as purely electrical signals. Namely, the polarizationdirection of the ferroelectric material is detected purely electrically.

As described above, in this embodiment, the series resonant circuitincluding the capacitor C formed directly below the probe 11 is providedand an alternating electric field is applied to the medium 10 while areference phase signal V10(t) is applied to the series resonant circuitto extract a resonant signal V11(t) from a connection point between theinductor L and (actually, the probe 11) the capacitor C of the seriesresonant circuit, and a change in the phase of the resonant signalV11(t) as an alternating electric field is applied to the medium 10 isextracted through comparison with the phase of the reference phasesignal V10(t), thereby performing detection of the direction ofpolarization of the medium 10, i.e., reproduction of data recorded onthe medium 10. The polarization direction of the medium 10 can also bedetected by converting a change of the capacitance due to application ofan alternating electric field into a phase change and extracting thephase change in the above manner. This eliminates the need for the FMdemodulator 30 and simplifies the configuration of the detection device,compared to the detection devices of the first and second embodiments.The detection device of this embodiment using such a detection principlecan also remove a noise component generated due to application of ahigh-frequency alternating electric field using the cancel signal V5(t),similar to the detection devices of the first and second embodiments.

Although this embodiment has been described with reference to the casewhere a series resonant circuit is formed through the inductor L and thecapacitor C which is formed directly below the probe, a parallelresonant circuit may also be formed through the inductor L and thecapacitor C.

What is claimed is:
 1. A device for detecting the direction ofpolarization of a ferroelectric material, the device comprising at leastone probe disposed in contact with or near a surface of a ferroelectricand an electric field applying means for providing an electric fieldsignal to the ferroelectric and applies an alternating electric field toa capacitor component formed in the ferroelectric directly below theprobe, wherein the device detects a direction of polarization of theferroelectric directly below the probe based on a capacitance change ofthe capacitor component as the alternating electric field is applied tothe capacitor component, the device further comprising: a demodulationmeans for generating a detection signal having a signal levelcorresponding to the capacitance change of the ferroelectric as thealternating electric field is applied from a measurement signal providedthrough the probe; a synchronous detection means for performingsynchronous detection of the detection signal based on a synchronoussignal and generates a polarization direction detection signalcorresponding to the polarization direction of the ferroelectric; and apseudo-noise signal generation means for generating a pseudo-noisesignal including a frequency component identical to a frequency of theelectric field signal, wherein the demodulation means includes a noisecomponent removal means for removing a noise component included in themeasurement signal through signal arithmetic processing with thepseudo-noise signal.
 2. The device according to claim 1, wherein thedemodulation means includes: an oscillation loop that includes the probeand the capacitor component and generates an oscillation signal at afrequency corresponding to capacitance of the capacitor component; and afrequency detector that generates a frequency detection signal having asignal level corresponding to the frequency of the oscillation signal,wherein the noise component removal means includes an arithmetic meansfor subtracting the pseudo-noise signal from the frequency detectionsignal.
 3. The device according to claim 2, wherein the demodulationmeans further includes a band pass filter that has a pass band whosecentral frequency is equal to the frequency of the electric fieldsignal.
 4. The device according to claim 1, wherein the demodulationmeans includes a control loop including: an oscillation loop thatincludes the probe and the capacitor component and generates anoscillation signal at a frequency corresponding to capacitance of thecapacitor component; and a voltage control oscillator that generates alocal oscillation signal at a frequency corresponding to a controlsignal provided to the voltage control oscillator; a mixer that mixesthe oscillation signal with the local oscillation signal and convertsthe oscillation signal into a low frequency signal; a frequency detectorthat generates a frequency detection signal having a signal levelcorresponding to a frequency of the low frequency signal; and acontroller that generates an output signal having a signal levelcorresponding to a deviation of the frequency detection signal from atarget level, wherein the noise component removal means includes anarithmetic means for providing a signal, obtained by inputting thepseudo-noise signal to the control loop, as the control signal to thevoltage control oscillator.
 5. The device according to claim 1, whereinthe demodulation means includes: a resonant circuit including thecapacitor component and an inductor component coupled to the probe; areference phase oscillator that applies a high frequency alternatingcurrent signal to the resonant circuit; and a phase comparator thatgenerates a phase difference signal having a signal level correspondingto a phase difference between the high frequency alternating currentsignal and a resonant signal generated by resonance through the inductorcomponent and the capacitor component due to the application of the highfrequency alternating current signal to the resonant circuit, whereinthe noise component removal means includes an arithmetic means forsubtracting the pseudo-noise signal from the phase difference signal. 6.The device according claim 1, wherein the electric field signal and thepseudo-noise signal are each a sinusoidal wave having a single frequencycomponent.
 7. The device according to claim 1, wherein the electricfield signal, the pseudo-noise signal, and the synchronous signal aregenerated based on a common reference frequency signal that oscillatesat a frequency identical to the frequency of the electric field signal.8. The device according to claim 7, wherein the pseudo-noise signalgeneration means includes: a phase adjuster that adds a delay time tothe reference frequency signal to change a phase of the referencefrequency signal; a band pass filter that has a pass band whose centralfrequency is equal to the frequency of the electric field signal andblocks frequency components, other than the pass band, of the referencefrequency signal; and an amplitude adjuster that changes an amplitude ofthe reference frequency signal.
 9. The device according to claim 8,wherein phase and amplitude of the pseudo-noise signal are variable. 10.A method for detecting the direction of polarization of a ferroelectricmaterial, wherein at least one probe is disposed in contact with or neara surface of a ferroelectric, an electric field signal is provided tothe ferroelectric, an alternating electric field is applied to acapacitor component formed in the ferroelectric directly below theprobe, and a direction of polarization of the ferroelectric directlybelow the probe is detected based on a capacitance change of thecapacitor component as the alternating electric field is applied to thecapacitor component, the method comprising: a demodulation processincluding generating a detection signal having a signal levelcorresponding to the capacitance change of the ferroelectric as thealternating electric field is applied from a measurement signal providedthrough the probe; a synchronous detection process including performingsynchronous detection of the detection signal based on a synchronoussignal and generating a polarization direction detection signalcorresponding to the polarization direction of the ferroelectric; and apseudo-noise signal generation process including generating apseudo-noise signal including a frequency component identical to afrequency of the electric field signal, wherein the demodulation processincludes a noise component removal process including removing a noisecomponent included in the measurement signal through signal arithmeticprocessing with the pseudo-noise signal.
 11. A device for detecting thedirection of polarization of a ferroelectric material, the devicecomprising at least one probe disposed in contact with or near a surfaceof a ferroelectric and an electric field applying part that provides anelectric field signal to the ferroelectric and applies an alternatingelectric field to a capacitor component formed in the ferroelectricdirectly below the probe, wherein the device detects a direction ofpolarization of the ferroelectric directly below the probe based on acapacitance change of the capacitor component as the alternatingelectric field is applied to the capacitor component, the device furthercomprising: a demodulation part that generates a detection signal havinga signal level corresponding to the capacitance change of theferroelectric as the alternating electric field is applied from ameasurement signal provided through the probe; a synchronous detectionpart that performs synchronous detection of the detection signal basedon a synchronous signal and generates a polarization direction detectionsignal corresponding to the polarization direction of the ferroelectric;and a pseudo-noise signal generation part that generates a pseudo-noisesignal including a frequency component identical to a frequency of theelectric field signal, wherein the demodulation part includes a noisecomponent removal part that removes a noise component included in themeasurement signal through signal arithmetic processing with thepseudo-noise signal.
 12. The device according to claim 11, wherein thedemodulation part includes: an oscillation loop that includes the probeand the capacitor component and generates an oscillation signal at afrequency corresponding to capacitance of the capacitor component; and afrequency detector that generates a frequency detection signal having asignal level corresponding to the frequency of the oscillation signal,wherein the noise component removal part includes an arithmetic partthat subtracts the pseudo-noise signal from the frequency detectionsignal.
 13. The device according to claim 12, wherein the demodulationpart further includes a band pass filter that has a pass band whosecentral frequency is equal to the frequency of the electric fieldsignal.
 14. The device according to claim 11, wherein the demodulationpart includes a control loop including: an oscillation loop thatincludes the probe and the capacitor component and generates anoscillation signal at a frequency corresponding to capacitance of thecapacitor component; and a voltage control oscillator that generates alocal oscillation signal at a frequency corresponding to a controlsignal provided to the voltage control oscillator; a mixer that mixesthe oscillation signal with the local oscillation signal and convertsthe oscillation signal into a low frequency signal; a frequency detectorthat generates a frequency detection signal having a signal levelcorresponding to a frequency of the low frequency signal; and acontroller that generates an output signal having a signal levelcorresponding to a deviation of the frequency detection signal from atarget level, wherein the noise component removal part includes anarithmetic part that provides a signal, obtained by inputting thepseudo-noise signal to the control loop, as the control signal to thevoltage control oscillator.
 15. The device according to claim 11,wherein the demodulation part includes: a resonant circuit including thecapacitor component and an inductor component coupled to the probe; areference phase oscillator that applies a high frequency alternatingcurrent signal to the resonant circuit; and a phase comparator thatgenerates a phase difference signal having a signal level correspondingto a phase difference between the high frequency alternating currentsignal and a resonant signal generated by resonance through the inductorcomponent and the capacitor component due to the application of the highfrequency alternating current signal to the resonant circuit, whereinthe noise component removal part includes an arithmetic part thatsubtracts the pseudo-noise signal from the phase difference signal. 16.The device according to claim 11, wherein the electric field signal andthe pseudo-noise signal are each a sinusoidal wave having a singlefrequency component.
 17. The device according to claim 11, wherein theelectric field signal, the pseudo-noise signal, and the synchronoussignal are generated based on a common reference frequency signal thatoscillates at a frequency identical to the frequency of the electricfield signal.
 18. The device according to claim 17, wherein thepseudo-noise signal generation part includes: a phase adjuster that addsa delay time to the reference frequency signal to change a phase of thereference frequency signal; a band pass filter that has a pass bandwhose central frequency is equal to the frequency of the electric fieldsignal and blocks frequency components, other than the pass band, of thereference frequency signal; and an amplitude adjuster that changes anamplitude of the reference frequency signal.
 19. The device according toclaim 18, wherein phase and amplitude of the pseudo-noise signal arevariable.